OpenML

Ensembling Tabular Foundation Models: A Diversity Ceiling and a Calibration Trap

This paper benchmarks six ensemble strategies across six tabular foundation models (TFMs) on 153 OpenML classification tasks, finding that ensembling provides minimal gains over the best single TFM. The best ensemble strategy (two-level cascade stacking) achieves only +0.18% accuracy improvement at 253× the compute cost. A key finding is that logistic-regression meta-learner stacking improves accuracy while severely degrading calibration (log-loss), because sharpening class boundaries destroys probability estimates. The authors recommend greedy ensemble selection as the practical default.